Phillip G. Jellyman

Dual influences of ecosystem size and disturbance on food chain length in streams

The number of trophic transfers occurring between basal resources and top predators, food chain length (FCL), varies widely in the worlds ecosystems for reasons that are poorly understood, particularly for stream ecosystems. Available evidence indicates that FCL is set by energetic constraints, environmental stochasticity, or ecosystem size effects, although no single explanation has yet accounted for FCL patterns in a broad sense. Further, whether environmental disturbance can influence FCL has been debated on both theoretical and empirical grounds for quite some time. Using data from sixteen South Island, New Zealand streams, we determined whether the so-called ecosystem size, disturbance, or resource availability hypotheses could account for FCL variation in high country fluvial environments. Stable isotope-based estimates of maximum trophic position ranged from 2.6 to 4.2 and averaged 3.5, a value on par with the global FCL average for streams. Model-selection results indicated that stream size and disturbance regime best explained across-site patterns in FCL, although resource availability was negatively correlated with our measure of disturbance; FCL approached its maximum in large, stable springs and was <3.5 trophic levels in small, fishless and/or disturbed streams. Community data indicate that size influenced FCL, primarily through its influence on local fish species richness (i.e., via trophic level additions and/or insertions), whereas disturbance did so via an effect on the relative availability of intermediate predators (i.e., predatory invertebrates) as prey for fishes. Overall, our results demonstrate that disturbance can have an important food web-structuring role in stream ecosystems, and further imply that pluralistic explanations are needed to fully understand the range of structural variation observed for real food webs.

Heavy metals: confounding factors in the response of New Zealand freshwater fish assemblages to natural and anthropogenic acidity

Acidification of freshwaters is a global phenomenon, occurring both through natural leaching of organic acids and through human activities from industrial emissions and mining. The West Coast of the South Island, New Zealand, has both naturally acidic and acid mine drainage (AMD) streams enabling us to investigate the response of fish communities to a gradient of acidity in the presence and absence of additional stressors such as elevated concentrations of heavy metals. We surveyed a total of 42 streams ranging from highly acidic (pH 3.1) and high in heavy metals (10 mg L(-)(1) Fe; 38 mg L(-)(1) Al) to circum-neutral (pH 8.1) and low in metals (0.02 mg L(-)(1) Fe; 0.05 mg L(-)(1) Al). Marked differences in pH and metal tolerances were observed among the 15 species that we recorded. Five Galaxias species, Anguilla dieffenbachii and Anguillaaustralis were found in more acidic waters (pH<5), while bluegill bullies (Gobiomorphus hubbsi) and torrentfish (Cheimarrichthys fosteri) were least tolerant of low pH (minimum pH 6.2 and 5.5, respectively). Surprisingly, the strongest physicochemical predictor of fish diversity, density and biomass was dissolved metal concentrations (Fe, Al, Zn, Mn and Ni) rather than pH. No fish were detected in streams with dissolved metal concentrations >2.7 mg L(-)(1) and nine taxa were only found in streams with metal concentrations <1 mg L(-)(1). The importance of heavy metals as critical drivers of fish communities has not been previously reported in New Zealand, although the mechanism of the metal effects warrants further study. Our findings indicate that any remediation of AMD streams which seeks to enable fish recolonisation should aim to improve water quality by raising pH above approximately 4.5 and reducing concentrations of dissolved Al and Fe to <1.0 mg L(-)(1).

Does one size fit all? An evaluation of length–weight relationships for New Zealand's freshwater fish species

Length–weight relationships are a fundamental tool for assessing populations and communities in fisheries science. Many researchers have collected length–weight data throughout New Zealand, yet parameters describing these relationships remain unpublished for many species of freshwater fish. We compiled 285,124 fish records from researchers and institutions across New Zealand to parameterise length–weight equations, using both power and quadratic models, for 53 freshwater species belonging to 13 families. The influence of location and sex on length–weight relationships was also assessed. Location, in particular, generated different length–weight relationships for 65% of the species examined. Length–weight equations were validated by comparing predicted weights against independently measured weights from 25 electrofished sites across New Zealand and the equations were highly accurate (R2>0.99). Recommendations are made about how to robustly apply this new resource which should assist freshwater fisheries researchers throughout New Zealand.

Controlling the Invasive Diatom Didymosphenia geminata : An Ecotoxicity Assessment of Four Potential Biocides

In 2004, an invasive mat-forming freshwater diatom, Didymosphenia geminata (didymo), was found in New Zealand causing concern with regard to potential consequences for local freshwater ecosystems. A four-stage research program was initiated to identify methods to control D. geminata. This article reports the results of Stage 2, in which four potential control compounds [Gemex™ (a chelated copper formulation), EDTA, Hydrothol®191, and Organic Interceptor™ (a pine oil formulation)] selected in Stage 1 were evaluated for their biocidal efficacy on D. geminata and effects on non-target organisms using both artificial stream and laboratory trials. Artificial stream trials evaluated the mortality rates of D. geminata and fishes to three concentrations of the four biocides, whereas laboratory toxicity trials tested the response of green alga and cladocera to a range of biocide concentrations and exposure times. In artificial stream trials, Gemex and Organic Interceptor were the most effective biocides against D. geminata for a number of measured indices; however, exposure of fishes to Organic Interceptor resulted in high mortality rates. Laboratory toxicity testing indicated that Gemex might negatively affect sensitive stream invertebrates, based on the cladoceran sensitivity at the proposed river control dose. A decision support matrix evaluated the four biocides based on nine criteria stipulated by river stakeholders (effectiveness, non-target species impacts, stalk removal, degradation profile, risks to health and safety, ease of application, neutralization potential, cost, and local regulatory requirements) and Gemex was identified as the product warranting further refinement prior to an in-river trial.

Pulse‐dose application of chelated copper to a river for Didymosphenia geminata control: Effects on macroinvertebrates and fish

A 1-h pulse-dose of a chelated Cu formulation (Gemex™; New Zealand) was applied to a river to test efficacy against the invasive mat-forming diatom Didymosphenia geminata (didymo) and to provide information on nontarget species effects that could not be adequately predicted from laboratory and experimental mesocosm studies. Intensive sampling allowed characterization of doses achieved at multiple downstream locations, and concurrent application of rhodamine dye allowed quantification of dispersion, adsorption, and dilution processes. The target dose of 10 to 20 mg Cu/L for 60 min was achieved at least 0.9 km downstream at sites with contrasting levels of didymo mat development. Adsorptive losses of Gemex were 12%/km where didymo was mostly nonvisible and approximately 36%/km where substantial didymo mats were present. At 0.9 km downstream, Cu concentrations peaked at 12 mg/L, and didymo was <5% viable (down from 65-72%) for ≥21 d posttreatment. Viability data indicate that elimination of nonvisible infestations is possible and that suppression of early-stage infestations (≤40% cover, ≤4.5 mm thick) could be achieved after repeated applications. After a single Gemex application, no significant accumulation of Cu was noted in the sediments six weeks posttreatment, but Cu concentrations remained high in algal mats (109-367 mg/kg dry wt). Long-term effects on the nontarget algal, invertebrate, or fish communities were minimal, although significant localized trout mortalities, not predicted by prior laboratory exposures, occurred on the treatment day. Extended Gemex exposure in low-hardness waters might have caused the mortalities, although changes in chelated Cu speciation also possibly contributed. The present study integrates effects on resident biota with dosage data, including changes in pH, in a natural waterway.

The role of dams in altering freshwater fish communities in New Zealand

Dams are important structures for generating electricity and irrigation, but they often severely modify river hydrology and geomorphology and thus impact freshwater communities. These effects are broadly predictable and therefore, at a catchment scale, biological communities should respond in some predictable manner. We interrogated the New Zealand Freshwater Fish Database to determine whether dams have predictable catchment-scale effects on fish assemblages. Seven selection criteria were applied to over 2000 dams, resulting in 30 dams being considered suitable for analysis. Comparisons were made above and below dams and with data from undammed sites which showed that community composition was significantly different above and below dams. Sites above dams had lower species richness, a lower percentage of diadromous species and a higher percentage of exotic species compared to below dams. None of these differences were present at undammed sites. In summary, dams restrict the movement of diadromous fish generally resulting in their reduction or loss from above-dam habitats and also create artificial lentic ecosystems that exotic species can successively exploit. These changes to fish communities were found across New Zealand and should be viewed as a general consequence of any new dam construction unless fish management and conservation measures are undertaken.

Increases in disturbance and reductions in habitat size interact to suppress predator body size.

Food webs are strongly size-structured so will be vulnerable to changes in environmental factors that affect large predators. However, mechanistic understanding of environmental controls of top predator size is poorly developed. We used streams to investigate how predator body size is altered by three fundamental climate change stressors: reductions in habitat size, increases in disturbance and warmer temperatures. Using new survey data from 74 streams, we showed that habitat size and disturbance were the most important stressors influencing predator body size. A synergistic interaction between that habitat size and disturbance due to flooding meant the sizes of predatory fishes peaked in large, benign habitats and their body size decreased as habitats became either smaller or harsher. These patterns were supported by experiments indicating that habitat-size reductions and increased flood disturbance decreased both the abundance and biomass of large predators. This research indicates that interacting climate change stressors can influence predator body size, resulting in smaller predators than would be predicted from examining an environmental factor in isolation. Thus, climate-induced changes to key interacting environmental factors are likely to have synergistic impacts on predator body size which, because of their influence on the strength of biological interactions, will have far-reaching effects on food-web responses to global environmental change.

Variable survival across low pH gradients in freshwater fish species

A series of 14 day experiments was conducted on five common New Zealand fish species (redfin bully Gobiomorphus huttoni, inanga Galaxias maculatus, brown trout Salmo trutta, longfin eel Anguilla dieffenbachii and koaro Galaxias brevipinnis) to assess the effect of pH on survival and changes in body mass. No species survived in water of pH <4 although there was 100% survival of all adults at pH 4.5, G. maculatus larvae were also tested and had high mortality at this pH. Results suggest that adults are tolerant of low-pH waters; however, successful remediation of anthropogenically acidified streams will require an understanding of the susceptibility to low pH on different life cycle stages.

Capacity to support predators scales with habitat size

Metabolic scaling means that reductions in habitat size that decrease predator body size will also reduce predator biomass that can be supported. Habitat reduction could drive biodiversity loss if the capacity of food webs to support predators is undermined by habitat-size constraints on predator body size. Assuming that (i) available space restricts predator body size, (ii) mass-specific energy needs of predators scale with their body size, and (iii) energy availability scales with prey biomass, we predicted that predator biomass per unit area would scale with habitat size (quarter-power exponent) and prey biomass (three-quarter–power exponent). We found that total predator biomass scaled with habitat size and prey resources as expected across 29 New Zealand rivers, such that a unit of habitat in a small ecosystem supported less predator biomass than an equivalent unit in a large ecosystem. The lower energetic costs of large body size likely mean that a unit of prey resource supports more biomass of large-bodied predators compared to small-bodied predators. Thus, contracting habitat size reduces the predator mass that can be supported because of constraints on predator body size, and this may be a powerful mechanism exacerbating reductions in biodiversity due to habitat loss.

The effect of ramp slope and surface type on the climbing success of shortfin eel (Anguilla australis) elvers

Instream structures such as dams, weirs and culverts reduce diadromous fish (e.g. freshwater eels) biodiversity and abundance by restricting upstream migrations. Fish passes are used to facilitate fish passage around instream structures, but limited information is available about design aspects that influence the ability of fish to negotiate the structures. We used experimental ramps to evaluate the effects of surface type and slope on the climbing success of shortfin eel (Anguilla australis) elvers (total length <155mm). Climbing success was examined for three surface types (smooth plastic, sand and gravel, Miradrain) at slopes of 30, 50 and 70°. Elver climbing ability differed among substrates, with the highest success observed on the Miradrain surface (a moulded plastic drainage product). Climbing success decreased with increasing ramp slope. The mean length of elvers successfully negotiating the ramps was longer than that of elvers that failed to climb because only larger elvers could climb the Miradrain surface as ramp slope increased. Results indicated that the optimal ramp-designed passage for climbing species would be lined with Miradrain and set at an angle close to 30° and not exceeding 50°. Elver passage will be detrimentally affected by steep surfaces, which will be exacerbated if suboptimal ramp-surface types are used.